Color-plus-clear coating systems involving the application of a colored or pigmented base coat to a substrate, followed by the application of a transparent or clear topcoat to the base coat, have become conventional as original finishes for automobiles. The color-plus-clear systems have outstanding gloss and distinctness of image, and the clear coat is particularly important for these properties.
In recent years, high solids liquid coatings and powder coatings have become increasingly popular because these coatings are inherently low in volatile organic content (VOC), which significantly reduces air emissions during the application process. Epoxy condensation polymers, vinyl chloride polymers, and acrylic resins have been used as the main film-forming polymers for these coatings. The advantage of the acrylic resin system over the epoxy condensation polymers and vinyl chloride polymers is that acrylic resins provide superior outdoor durability, are more resistant to heat, and have better solvent and chemical resistance. Glycidyl functional acrylic resins provide such desired properties.
Additional requirements in the automotive coatings market include good exterior durability, particularly upon exposure to extreme conditions, acid etch and water spot resistance, and excellent gloss and appearance. Currently, these properties can be achieved using epoxy-acid cured coatings.
Although epoxy-acid systems are known for powder coatings, they are presently not widely used commercially for automotive OEM clear coatings in color-plus-clear composite systems because such coatings can exhibit inferior appearance properties, such as yellowing, poor flow, and poor cured film properties. Yellowing is often due to the thermal oxidation of amine functional catalysts present in the coatings for curing purposes. Yellowing may also be caused by overbake conditions and interactions with underlayers.
It would be desirable to provide curable film-forming compositions and color-plus-clear composite coating compositions that overcome the drawbacks of the prior art by exhibiting reduced yellowing, while maintaining other properties such as humidity resistance, acid etch and water spot resistance, and exterior durability.